Ultrastructural localization of calretinin immunoreactivity in lobule V of the rat cerebellum.

Ultrastructural localization of a calcium-binding protein, calretinin, in the cerebellar cortex of the rat was examined with an immunoperoxidase method. The cerebellar lobule V was investigated in detail. In the molecular layer, calretinin immunoreactivity was found in parallel fibers and in their varicosities that formed synapses with unlabeled Purkinje cell dendritic thorns and Golgi cell dendrites. In the granular layer, most granule cells were immunostained for calretinin. Within immunostained granule cells, the labels were found in the cytoplasmic matrix of both the perikaryon and primary dendrite, and in euchromatin patches of the nucleus. Some mossy fiber terminals also stained for calretinin. In the white matter, a few axons were labeled for calretinin and surrounded by unstained myelin sheath. It is suggested that calretinin may play a role at the presynaptic sites of the parallel and mossy fiber terminals.